WDM optical communication systems are today's leading edge state-of-the-art. In a very near future, the high-data-rate WDM channels will have a predetermined spectral position (proposed ITU-Grid). When a WDM wavelength grid standard will be settled upon, it will be possible to act on each discrete channels independently.
For high-data-rate (greater than 1 Gbps per channel) optical communication systems, chromatic dispersion has a major impact on the transmission performance. Chromatic dispersion results in the spectral components of a laser light signal having different transmission velocities in a same light guide medium, that is the index of refraction changes as a function of wavelength of the light.
A digital WDM optical signal comprises a number of channels each assigned a predetermined optical wavelength or frequency. For each channel, a transmitting optical source, such as a laser, operates at the predetermined channel wavelength and sends through a waveguide a series of pulses encoding the data. In practice, an optical source does not emit light at a precise single wavelength, but rather the energy of the optical source is output within a narrow range of wavelengths around the predetermined channel wavelength. In a dispersive lightguide, such as an optical fiber, a "clean" squar.ang. pulse input into a fiber will exit from the optic fiber kilometers away with a certain degree of spread in time due to dispersion. Such spilling over of signal from one bit time slot to another can lower the pulse intensity level for the one bit, thus reducing the threshold for reliable detection, and also cause otherwise blank or zero level bit time slots to contain signal, thus requiring a raising of the threshold for zero detection. Even if a detector at the receiver end could precisely detect only an exact wavelength and thus be unaffected by dispersion, the signal energy at the precise wavelength from the optical source would be practically very low. Therefore, as bitrates are increased in optical communications, it is important to provide dispersion compensation, which improves the quality of bit detection and transmission speed.
Chromatic dispersion can be avoided by using fibers having nearly zero dispersion around the wavelength region to be used. However, dispersion has the advantage of decreasing non-linear interactions between the WDM channels that could create major signal distortion. Thus, managing chromatic dispersion with the help of dispersion compensators appears to be a better solution. Chirped fiber Bragg gratings (FBGs) have proven to be efficient devices to provide dispersion compensation. FBGs may be linearly or non-linearly chirped. An in-fiber Bragg grating is created by providing a periodic modulation in the fiber (typically of the index of refraction), the periodicity of which causes reflection substantially at an exact wavelength. A chirped fiber Bragg grating is an extended Bragg grating in which the periodicity changes over the extent of the Bragg grating to reflect different wavelengths at different points in the grating. As a result, the reflected signal has various wavelength components delayed according to wavelength, and the chirped grating can be constructed to cause compensation of dispersion within a WDM optical channel, i.e. slowing down wavelength components which have faster velocities with respect to wavelength components having slower velocities in the fiber.
Chirped Bragg gratings, depending on the degree of dispersion being compensated, may have a length of about 10 cm in fiber for correcting dispersion of a single optical frequency channel. For dispersion compensation of a WDM signal having a plurality of optical channels, a plurality of chirped Bragg gratings, one for each optical channel to be dispersion compensated, is typically used. The channel wavelengths may be standardized ones, as for example those of the proposed ITU grid. A single continuous chirped Bragg grating having a length of about 100 cm could also be feasible for compensating a plurality of channels in a WDM signal. While more complicated to manufacture, such a continuous in-fiber chirped Bragg grating has the advantage that a reconfiguration of the channel spacing in the WDM signal (e.g. to increase a number of channel within a certain optical bandwidth) would not require replacement of the fiber Bragg grating.
In-fiber Bragg gratings and their manufacture are known in the art, as disclosed for example in Appl. Phys. Lett. 62(10), Mar. 8, 1993 at page 1035, "Bragg gratings fabricated in a monomode photosensitive optical fiber by UV exposure through a phase mask" by Hill et al., and in IEE Conference Publication No. 448, ECOC '97, Sep. 22-25, 1997, page 195, "SIMPLE AND FLEXIBLE TECHNIQUE FOR SPECTRALLY DESIGNING ALL-FIBRE FILTER AND APODIZING FIBRE GRATINGS" by Guy et al. The architecture of a Bragg grating dispersion compensator is known in the art, as for example in Optics Letters, Sep. 1, 1994, vol. 19, no. 17, page 1314 "Chirped in fiber Bragg gratings for compensation of optical-fiber dispersion", by Hill et al. Using such a grating for dispersion compensation in association with an EDFA has been proposed in Electronic Letters, Dec. 7 1995, Vol. 31, No. 25, page 2149, "Multichannel equalised and stabilised gain amplifier for WDM transmissions" by Delavaque et al. A plurality of in-fiber signal gratings are provided spaced apart from one another. A Bragg grating in a fiber is provided for each wavelength of light used for optical communication in the fiber. An optical circulator feeds the input signal into the fiber provided with the gratings and then feeds the reflected conditioned channel signals into the output port. These multiple Bragg gratings not only compensate the dispersion over each WDM channel, they also are able to equalize the relative gain of these channels and thus to condition the WDM channels. The gain is equalized by different Bragg gratings having different reflectivities, associated with different WDM channels.
All-optical amplifiers, namely erbium-doped optically pumped fiber amplifiers, used in optical data transmission systems, have the advantage of eliminating opto-electronic repeaters. Signal amplification is required to transmit optical signals over great distances due to fiber losses. When amplifying a WDM signal using an all-optical amplifier, the gain is not uniform with wavelength, thus it is different for the WDM channels. It is therefore advantageous to apply channel equalization so that the amplifier output has the desired signal intensity for each channel.
The combination of dispersion compensation and channel equalization is very advantageous in WDM optical transmission. Both dispersion compensation and channel equalization provide a conditioning of the optical signal that allows for better detection of the transmitted optical signal.
It is desirable to associate signal conditioning with a signal amplification since signal distortion, as signal attenuation, increases proportionally with the propagation distance in the guiding medium. Moreover, signal conditioning with fiber Bragg gratings is always associated with signal losses. The integration of multiple conditioning functions with an optical amplifier into one device allows the optimization of the optical amplifier considering the joint impact of the conditioning functions on the signal. For example, in order for the optical amplifier associated with the "signal conditioner" to have a small noise figure (signal-to-noise ratio deterioration due to the optical amplifier), the signal coming into the amplification stage should have minimal loss. Such a "signal conditioner"/optical amplifier combination with a low noise figure has not been adequately provided by known prior art apparatus.